Duration & elapsed time
Theory
Elapsed time is how much time passes between a start and a finish. The easy way to find it is to count on: to the next o’clock, then in whole hours, then the extra minutes. This Year 6 maths page shows how to find durations, finishing times and starting times, how to change between hours and minutes, and how to cross midday and midnight.
The duration (elapsed time) of an event is the time from its start to its finish.
There are \(60\) minutes in an hour and \(24\) hours in a day. Half an hour is \(30\) minutes; a quarter of an hour is \(15\) minutes.
Counting on: from 7:35 am, count \(25\) min to 8:00, then \(2\) h to 10:00, then \(20\) min to 10:20. That is \(2\) h \(45\) min.
Times are not decimals. You cannot subtract 7.35 from 10.20, because an hour has \(60\) minutes, not \(100\).
In 24-hour time, afternoon hours are \(12\) more: 2:35 pm is \(\text{14:35}\). Midnight is \(\text{00:00}\).
Time facts:
| Hours and minutes | Minutes |
|---|---|
| \(1\) h \(15\) min | \(75\) |
| \(1\) h \(30\) min | \(90\) |
| \(2\) h | \(120\) |
| \(2\) h \(50\) min | \(170\) |
| \(4\) h | \(240\) |
How to find a duration
- Count on from the start time to the next o'clock.
- Add whole hours up to the hour of the finish time.
- Add the extra minutes, then total the jumps.
How to find a finishing or starting time
- Finish = start + duration: add the hours, then the minutes, crossing the hour if needed.
- Start = finish − duration: count back.
- Check am/pm when you pass 12 o'clock.
| \(60 - 25\) | \(=\) | \(35\) |
| \(35 + 10\) | \(=\) | \(45\) |
\(35\) min to 3:00 pm, then \(10\) min: \(45\) minutes.
7:35 am to 8:00 am is \(25\) min, 8:00 am to 10:00 am is \(2\) h, and 10:00 am to 10:20 am is \(20\) min.
| \(25 + 20\) | \(=\) | \(45\text{ min}\) |
\(2\) h \(45\) min, as in the first diagram.
11:50 am \(+ 1\) h \(=\) 12:50 pm. Then \(+ 35\) min: \(10\) min to 1:00 pm and \(25\) min more.
1:25 pm.
\(80\) min \(= 1\) h \(20\) min. Count back: 4:10 pm \(\to\) 3:10 pm \(\to\) 2:50 pm.
2:50 pm.
(ii) Write \(230\) minutes in hours and minutes.
| \(3 \times 60 + 10\) | \(=\) | \(190\) |
| \(230 - 180\) | \(=\) | \(50\) |
(i) \(190\) minutes
(ii) \(3\) h \(50\) min
| \(20 + 120\) | \(=\) | \(140\) |
| \(140 + 305\) | \(=\) | \(445\) |
\(140\) min to midnight, then \(5\) h \(5\) min (\(305\) min): \(445\) minutes \(= 7\) h \(25\) min.
| \(25 + 120 + 10\) | \(=\) | \(155\) |
\(25\) min to \(\text{15:00}\), \(2\) h to \(\text{17:00}\), \(10\) min more: \(2\) h \(35\) min (\(155\) min).
| \(60 + 55\) | \(=\) | \(115\) |
| \(118 - 115\) | \(=\) | \(3\) |
Film B, by \(3\) minutes.
| \(40 + 5\) | \(=\) | \(45\) |
Times are not decimals. Count on: \(40\) min to 12:00 pm, then \(5\) min: \(45\) minutes.
Common pitfalls
Frequently asked questions
How many minutes are in an hour?
\(60\).
What is the easiest way to find elapsed time?
Count on: to the next o'clock, then whole hours, then the extra minutes.
How do I change hours to minutes?
Multiply the hours by \(60\) and add the extra minutes: \(3\) h \(10\) min \(= 190\) min.
How do I change minutes to hours and minutes?
Take away as many \(60\)s as you can: \(230\) min \(= 3\) h \(50\) min.
How do I find a time that crosses midnight?
Count on to midnight (12:00 am), then add the time after midnight.